In U.S. Pat. No. 4,081,520, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, there is described a process for the production of chlorine dioxide at high efficiency using sodium chlorate, sulfuric acid and methanol. The reaction medium from which the chlorine dioxide is formed is maintained at its boiling point, generally in the range of about 50.degree. to about 85.degree. C., under a subatmospheric pressure. The evaporated water serves to dilute the chlorine dioxide for removal from the reaction zone.
The mechanism of reaction is thought to be that chlorine, coproduced with the chlorine dioxide, is reduced by the methanol to chloride ions, which then react with the sodium chlorate. The reaction medium generally has a total acid normality greater than about 4.5 normal, which results in the formation of a sodium acid sulfate, usually sodium sesquisulfate, which precipitates from the reaction medium, after becoming saturated with the salt following start-up. The process equation may be represented as: ##STR1## This process is highly efficient in terms of the conversion of chlorate ions to chlorine dioxide and efficiencies well in excess of 90% can be achieved. The chlorine dioxide which results is virtually uncontaminated by chlorine, which is highly beneficial in the modern pulp mill environment. The process described in U.S. Pat. No. 4,081,520 has been widely adopted commercially and is known as the "R8" process.
As noted above, the by-product of this process is a sodium acid sulfate. This material may be used, as is, in the pulp mill as a make-up chemical for sulfur values lost from the mill. However, with the considerable tightening of the pulp mill environment in recent years, such sulfur losses have been significantly decreased, so that the requirement for make-up sodium sulfate also has declined to the stage where the material is an unwanted by-product.
Owing to the acidic nature of the by-product, sodium sesquisulfate cannot readily be disposed of in that form, requiring neutralization of the acid values prior to disposal. With the trend towards higher chlorine dioxide substitution for chlorine in many mills, the necessity to neutralize increasing quantities of sodium sesquisulfate may result in an imbalance of caustic and chlorine within the mill. The caustic demand increases while chlorine usage declines, resulting in increased costs to the mills. In addition, the lost acid values of the sodium sesquisulfate require that make-up sulfuric acid must be fed continuously to the chlorine dioxide generator to maintain the required acidity.
There have been prior suggestions to alleviate this problem in the R8 process and also in other chlorine dioxide generating processes which produce an acid sulfate by-product. For example, in U.S. Pat. No. 3,975,505 there is described the conversion of acid sulfate produced in a high acidity process for generating chlorine dioxide from sodium chlorate, sodium chloride and sulfuric acid, by passing crystalline by-product from the chlorine dioxide generator countercurrent to warm wash water in a washing column, which has the effect of not only freeing the crystalline material of entrained reaction medium but also converting the sodium acid sulfate to neutral sodium sulfate.
U.S. Pat. No. 4,325,934, assigned to the assignee hereof, describes contacting the solid phase by-product sodium acid sulfate from an R8 chlorine dioxide generator with a mixture of water and methanol to produce solid phase neutral sodium sulfate. This prior art reference describes the preferred use of weight ratios of water to sodium acid sulfate (calculated as Na.sub.3 H(SO.sub.4).sub.2) of about 0.6:1 to about 0.8:1 and of methanol to sodium acid sulfate (calculated as Na.sub.3 H(SO.sub.4).sub.2) of about 0.3:1 to about 0.8:1. Acid values recovered by this process are recycled to the chlorine dioxide generator, while excess methanol is stripped from the acidic solution.
Canadian Patent No. 1,118,184 granted to the assignee hereof, describes a procedure in which the solid phase by-product from the R8 process is contacted with warm water to effect conversion of by-product sodium sesquisulfate to neutral sodium sulfate in a multi-stage decantation-washing operation.
None of these procedures has proved to be commercially attractive, for a variety of reasons. The procedure described in U.S. Pat. No. 3,975,505 refers to a different process for forming chlorine dioxide as well as significantly increasing the evaporative load on the generator, U.S. Pat. No. 4,325,934 requires a costly stripping operation with respect to the excess of methanol employed and the equipment described in Canadian Patent No. 1,118,184 has been found to plug frequently and to add approximately three to four tonnes of water per tonne of chlorine dioxide generated to the evaporative load of the generator.